The present invention relates generally to a field of optical information processing and more particularly to a method and system for detecting ultrasonic displacements in a material under test utilizing a time-varying output pulse of a laser beam.
In recent years, the use of advanced composite structures has experienced tremendous growth in the aerospace, automotive, and many other commercial industries. While composite materials offer significant improvements in performance, they require strict quality control procedures in the manufacturing processes. Specifically, non-destructive evaluation (xe2x80x9cNDExe2x80x9d) methods are required to assess the structural integrity of composite structures, for example, to detect inclusions, de-laminations and porosities. Conventional NDE methods, however, are very slow, labor-intensive, and costly. As a result, testing procedures adversely increase the manufacturing costs associated with composite structures.
Various systems and techniques have been proposed to assess the structural integrity of composite structures. One method to generate and detect ultrasound using lasers discloses the use of a first modulated, pulsed laser beam for generating ultrasound on a work piece and a second pulsed laser beam for detecting the ultrasound. Phase modulated light from the second laser beam is then demodulated to obtain a signal representative of the ultrasonic motion at the surface of the work piece.
Another method to generate and detect ultrasound using lasers discloses the use of a laser to detect deformations of a oscillatory or transient nature on a material under test surface. The deformations on the material under test surface can be produced by an ultrasound wave or other excitation. Light from the laser is scattered by the deformations, some of which light is collected by collecting optics and transmitted via a fiber optic to a beam splitter which deflects a small portion of the collected light to a reference detector and delivers the remaining portion of the light to a confocal Fabry-Perot interferometer, which generates an output signal indicative of the deformations on the material under test surface. The reference detector measures the intensity of the scattered laser light at the input of the interferometer to generate a reference signal. A stabilization detector measures the intensity of the scattered laser light at the output of the interferometer to generate a prestabilization signal. The ratio of the reference signal to the prestabilization signal is used to generate a final stabilization signal which drives a piezoelectric pusher inside the interferometer to adjust its resonant frequency.
The advanced composite structures often attenuate ultrasound within the composite materials. It would be desirable to have a system capable of expanding the dynamic range of ultrasound detection in an attenuative material such as advanced composites.
The above-referenced methods attempt to reduce the noise associated with the detection schemes. However, the methods disclosed do not explore expanding and improving the dynamic range of ultrasound detection in attenuative materials.
Therefore, there is a need has arisen for a method and system of ultrasonic laser detection that overcomes the disadvantages and deficiencies of the prior art. Namely, such a system should be able to extend the dynamic range of ultrasound detection in an attenuative material.
The present invention provides a method and system for detecting ultrasonic displacements at a remote target under test utilizing a laser beam that substantially eliminates or reduces disadvantages and problems associated with previously developed ultrasonic detection systems.
More specifically, the present invention provides a system for detecting ultrasonic displacements at a remote target with a laser beam having a time dependent pulse profile. The system and method for improving the dynamic range of laser detected ultrasonic in attenuative materials includes a seed laser light source. This laser source produces a laser which is modulated by an assembly placed in the laser beam""s path. The modulated laser has a time-dependent pulse profile. Ultrasonics at the remote target further modulate, reflect and/or scatter the laser beam to produce phase-modulated light. Optics collect this phase modulated light. An interferometer coupled to the collection optics demodulates the phase-modulated light and provide an output signal representative of the ultrasonics at the remote target.
A processor may be utilized to process output signal of the interferometer to obtain data representative of the ultrasonics.
Another embodiment of the present invention involves matching the time-dependent pulse profile of the detection laser beam to the attenuative properties of the remote target. Alternatively, the time-dependent pulse profile may be varied to increase the signal strength of the detected ultrasonics.
The present invention provides an important technical advantage by extending the dynamic range of a Laser UT system. Previous systems would synchronize the generation of the ultrasonic event with the peak of the detection laser to maximize signal-noise-ratio without regard for potential dynamic range improvements based on exploiting non-uniform illumination profiles, while the present invention provides that the use of time-dependent detection laser illumination profiles can be used to both optimize signal-noise-ratio and extend the dynamic range of the Laser UT systems.
Another technical advantage of the present invention is an extended dynamic range with which to detect ultrasound in the material under test and improved signal-to-noise ratio due to the time-varying pulse profiles of the detection laser.
Yet another technical advantage of the present invention is the ability to use a detection laser with lower output power. This allows the use of smaller collection optics and optical scanners. Additionally, the use of a lower power detection laser reduces the total power applied to the material under test and damage of the material under test.
Stored energy in amplifier can be extracted in an optimum way to match the properties of the material under test.